EP0736187B1 - Optical polarizer - Google Patents

Description

Technical Field
• The invention is an improved optical polarizer.
Background
• Optical polarizing film is widely used for glare reducing sunglasses,increasing optical contrast, and in Liquid Crystal Displays (LCD). The mostcommonly used type of polarizer used in these applications is a dichroicpolarizer. Dichroic polarizers are made by incorporating a dye into a polymersheet that is stretched in one direction. Dichroic polarizers can also be made byuniaxially stretching a semicrystalline polymer such as polyvinyl alcohol, thenstaining the polymer with an iodine complex or dichroic dye, or by coating apolymer with an oriented dichroic dye. These polarizers typically have anextinction ratio (the ratio of transmission of light polarized perpendicular to thestretch direction to the polarization parallel to the stretch direction) of over500:1. Dichroic polarizers normally has some absorption of light polarized inthe high transmission orientation. Losses in this orientation are typically10-20%.
• Commercial polarizers typically use polyvinyl alcohol as the polymermatrix for the dye, however other polymers can be used. US Patent 4,756,953describes the use of polyethylene naphthalate as the polymer matrix.
• Low profile, sheet form reflective polarizers are available that reflect onepolarization of light and transmit the other. These polarizers tend to be moreefficient in transmitting light of the high transmission polarization. This is due tothe use of a non-absorbing dielectric stack for polarizing light. These polarizerstend to have equal reflectivity for light irradiating the sheet from either side.These types of polarizers also tend to have some defects, such as leakage of light through localized areas of the sheet, and incomplete reflectivity of the highextinction polarization over the wavelength region of interest. This leakage oflight and incomplete reflectivity is often called iridescence.
• WO 94/11776 refers to a radiant energy conservation LCD-display, whichcomprises a filter located forward of a backlight for transmitting a filtered lightof specified frequencies and for reflecting out of band light rather than absorbingit. The filter includes an reflecting polarizer for transmitting a lighthaving a first polarization and reflecting a light having a second polarization.EP 0 488 544 refers to multi layer birefringent interference polarizers whichuses an nonabsorbing dielectric stack for polarizing light. DE-A-41 21 861 disclosesa birefringent interference polarizer, which exhibits high brightness.
• A classification of polarizers and a definition of wire-grid polarizers is providedin E. Hecht/A. Zajac handbook "Optics", Sixth printing, 1980, Addison-WesleyPublishing company, pages 225 to 230. Wire-grid polarizers are described in G.Bird and M. Parrish, "The Wire Grid as a Near-Infrared Polarizer", J. Opt. Soc.of Am.,Volume 50, pages 886 to 891, 1960.
Summary
• A reflective polarizer and a dichroic polarizer are combined to provide an improvedoptical polarizer. The reflective polarizer contains at least one birefringentmaterial. The dichroic and reflective polarizers are in close proximity toeach other, and are preferably bonded together to eliminate the air gap betweenthe polarizers. The combination of the two polarizers provides a hightransmission for light of a first polarization and high reflectivity for light of asecond, perpendicular polarization from the reflective polarizer side of the opticalpolarizer, and high transmission for light of the first polarization and highabsorption for light of the second, perpendicular polarization from the dichroic polarizer side. Iridescence as seen in transmission and when viewed in reflectionfrom the dichroic polarizer side is also reduced as compared to the reflectivepolarizer alone. This reduction in iridescence is useful in improving thecosmetic appearance of optical displays, the extinction ratio of optical polarizers,and the optical uniformity of a display. The combination of the two polarizersproduces antireflection on the side of the reflective polarizer in closeproximity to the dichroic polarizer.
• The increased extinction ratio and low reflectivity of the present optical polarizerallows use of a lower extinction ratio dichroic polarizer in applications requiringa given extinction ratio. By lowering the extinction ratio required of dichroicpolarizer the absorptive losses In the dichroic polarizer for transmittedrays can be reduced. Thus, the present optical polarizer has improved transmissiveextinction ratios for rays entering from either side of the present opticalpolarizer, low reflected intensity for rays partially transmitted by the dichroicpolarizer that are reflected by the reflective polarizer, and lower absorptivelosses as compared to a dichroic polarizer alone.
Brief Description of the Drawings
• The various objects, features and advantages of the present opticalpolarizer shall be better understood upon reading and understanding thefollowing detailed description and accompanying drawings in which:
• FIGURE 1 shows the present optical polarizer, including a reflectivepolarizer and a dichroic polarizer placed proximate thereto;
• FIGURE 2 shows a preferred multilayer reflective polarizer having adichroic polarizer bonded thereto;
• FIGURE 3 shows one embodiment of a display incorporating a reflectivepolarizer and dichroic polarizer;
• FIGURE 4 shows another emboidment of a display incorporating areflective polarizer and dichroic polarizer;
• FIGURE 5 shows another embodiment of a display incorporating twocombined reflective polarizers and dichroic polarizers;
• FIGURE 6 shows a liquid crystal display incorporating a reflectivepolarizer and a dichroic polarizer;
• Figure 7 shows a two layer stack of films forming a single interface.
• Figures 8 and 9 show reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.60.
• Figure 10 shows reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.0.
• Figures 11, 12 and 13 show various relationships between in-planeindices and z-index for a uniaxial birefringent system.
• Figure 14 shows off axis reflectivity versus wavelength for twodifferent uniaxial birefringent systems.
• Figure 15 shows the effect of introducing a y-index difference in abiaxial birefringent film.
• Figure 16 shows the effect of introducing a z-index difference in abiaxial birefringent film.
• Figure 17 shows a contour plot summarizing the information fromFigures 10 and 11;
• Figures 18-23 show optical performance of multilayer mirrors given inthe mirror Examples; and
• Figures 24-27 show optical performance of multilayer polarizers givenin the polarizer Examples.
Detailed Description
• PIG. 1 shows anoptical polarizer 10 that has two primary components.These are adichroic polarizer 11 and areflective polarizer 12. The two polarizersare aligned to provide maximum transmissivity. The combination of the twopolarizers provides a high transmission for light of a first polarization and highreflectivity for light of a second, perpendicular polarization from the reflcetivepolarizer side of the optical polarizer, and high transmission for light of the firstpolarization and high absorption for light of the second, perpendicular polarizationfrom the dichroic polarizer side.
• In use, the combined polarizers are illuminated on one or both of the outsidefacing surfaces. Ray 13 is of a polarization that is preferentially reflected byreflective polarizer 12 to formray 14. Light ofray 13 transmitted byreflectivepolarizer 12forms ray 15 which is attenuated bydichroic polarizer 11.Light ray16 which is perpendicularly polarized toray 13 is preferentially transmitted byreflective polarizer 12 and is slightly attenuated bydichroic polarizer 11. Ray 17 isof a polarization that is preferentially absorbed bydichroic polarizer 11, and whichis also preferably of the same polarization asray 13. The portion of light ofray 17which is transmitted bydichroic polarizer 11 is further attenuated by reflection offreflective polarizer 12 formingray 18 which is further absorbed bydichroicpolarizer 11.Light ray 19 which is polarized perpendicular toray 17, and which isof the same polarization asray 16, is preferentially transmitted by both dichroic andreflective polarizers 11 and 12, respectively.
• Thedichroic polarizer 11 is typically in close proximity to thereflectivepolarizer 12. Preferably they are bonded to each other to eliminate the air gapbetween the polarizers, as shown in FIG. 2.
• The preferred and illustrativereflective polarizer body 12 shown inFIG. 2 is made of alternating layers (ABABA...) of two different polymericmaterials. These are referred to as material "(A)" and material "(B)" throughoutthe drawings and description. The two materials are extruded together and theresulting multiple layer (ABABA...) material is stretched (5:1) along one axis(X) and is not stretched appreciably (1:1) along the other axis (Y). The X axis isreferred to as the "stretched" direction while the Y axis is referred to as the"transverse" direction.
• The (B) material has a nominal index of refraction (1.64 for example)which is not substantially altered by the stretching process. The (A) materialhas the property of having the index of refraction altered by the stretchingprocess. For example, a uniaxially stretched sheet of the (A) material will haveone index of refraction (1.88 for example) associated with the stretched directionand a different index of refraction (1.64 for example) associated with thetransverse direction. By way of definition, the index of refraction associatedwith an in-plane axis (an axis parallel to the surface of the film) is the effectiveindex of refraction for plane-polarized incident light whose plane of polarizationis parallel to that axis.
• Thus, after stretching, the multiple layer stack (ABABA...) of materialshows a large refractive index difference between layers (1.88 minus 1.64)associated with the stretched direction. While in the transverse direction, theassociated indices of refraction between layers are essentially the same (1.64 and1.64 in the example). These optical characteristics cause the multiple layerlaminate to act as a reflecting polarizer that will transmit the polarizationcomponent of the incident light which is correctly oriented with respect to theaxis 22.Axis 22 is defined as the transmission axis. The light which emergesfrom thereflective polarizer body 12 is referred to as having a first polarizationorientation.
• The light which does not pass through thereflective polarizer body 12 hasa polarization orientation orthogonal or perpendicular to the first orientation.Light exhibiting this polarization orientation will encounter the index difference which results in reflection of this light. This defines a so-called "extinction"axis24. In this fashion thereflective polarizer body 12 passes light having a selectedpolarization.
• The optical performance of thereflective polarizer body 12 depends inpart on the optical thicknesses of the various layers. Both thick film and thinfilm constructions are useful. If the layers exhibit optical thicknesses that aremany wavelengths of light long, then the optical properties are inherently broadband. If the layers have an optical thickness less than a wavelength of light, thenconstructive interference can be exploited to improve the optical performance ofthereflective polarizer body 12 at selected wavelengths.
• The manufacturing procedure described in the example can produceuniform layers that have an optical thickness that is less than the wavelength oflight over the visible spectrum. Constructive interference occurs if pairs oflayers (A,B) add to one half of the wavelength of the incident light(A+B=lambda/2). This half wavelength condition results in narrow bandconstructive interference at the design wavelength. Broad band opticalperformance can be achieved by laminating or otherwise coupling multiplenarrow band stacks. For example, afirst group 38 of layers having the samethickness (A+B=lambda/2) can be laminated to a second group 41 having adifferent thickness (A+B=lambda prime/2). For the sake of clarity only a smallnumber of layers are shown in FIG. 2, although typically hundreds of layers(ABAB...) may be stacked together to achieve efficient broad band response.Preferably thereflective polarizer body 12 should be designed to reflect light atall angles and wavelengths of interest.
• The manufacture of the multilayer film stack often times incorporates"skin" layers to surround the multilayer stack. Typically this skin layer iscomposed of either polymer "A" or polymer "B", and is thicker than thewavelength of light.
• The optical performance and uniformity of a reflective polarizer can beimproved by adding a dichroic polarizer proximate to at least one side of themultilayer stack, or by incorporating a dichroic polarizer into at least one of the layers in the multilayer stack. In such a configuration, thetransmission axis 27 ofthedichroic polarizer 11 is preferably aligned with thetransmission axis 22 of thereflective polarizer 12. When thedichroic polarizer 11 is on one side ofreflectivepolarizer 12, as shown in FIG. 1, the reflection oflight ray 17 on the dichroicpolarizer side will be reduced due to attenuation of reflectedray 18 bydichroicpolarizer 11 in comparison to the reflection ofray 17 offreflective polarizer 12withoutdichroic polarizer 11. The reflectivity ofray 13 offreflective polarizer 12 isnot substantially affected bydichroic polarizer 11. This produces anoptical polarizer10 which is antireflective on at least one side. Antireflection of one side of theoptical polarizer 10 is useful in displays, particularly in certain backlit displays wherethe reflected polarization can be used to increase the display brightness while theother side, the viewing side, of the polarizer must not reflect light. Iridescence asseen in transmission through either direction, and iridescence when viewed inreflection from the dichroic polarizer side are reduced by the addition ofdichroicpolarizer 11. This reduction in iridescence is useful in improving the cosmeticappearance of the display, the extinction ratio of the polarizer, and the opticaluniformity of the display.
• The configuration of dichroic and reflective polarizers shown in FIG. 1 createsa high efficiency optical polarizer. Combiningdichroic polarizer 11 withreflectivepolarizer 12 results in anoptical polarizer 10 which has a higher extinction ratio fortransmitted light than that which is achieved using the dichroic polarizer alone. Thisconfiguration also produces low reflectivity forray 17 from the dichroic polarizerside due to attenuation of reflectedray 18 bydichroic polarizer 11. For applicationsrequiring a given extinction ratio and high transmissivity, the increased extinctionratio and low reflectivity ofoptical polarizer 10 allows the use of adichroic polarizer11 which has a lower extinction of the first polarization than could otherwise be used.By lowering the extinction required ofdichroic polarizer 11, the absorptive losses inpolarizer 11 for transmittedrays 16 and 19 can be reduced. Thus, theopticalpolarizer 10 has improved transmissive extinction ratios forray pair 17 and 19 andray pair 13 and 16, low reflected intensity for reflectedray 18 off ofreflectivepolarizer 12, and lower absorptive losses than could be achieved using a dichroic polarizer alone. The preferred extinction for thedichroic polarizer 11 for use inliquid crystal displays is 10 to 99.99%, more preferred is 50 to 99%, more preferredis 60 to 97%, and most preferred is 70 to 95%. The preferred extinction for thereflective polarizer is 20 to 99.99%, more preferred is 50 to 99.9% and mostpreferred is 90 to 99%.
• Reflective polarizers may have some dielectric interference in the secondpolarization at either normal or off-normal angles, or both. This reflection maypresent problems due to reflected glare and attenuation of transmitted light of thesecond polarization. An efficient dichroic polarizer aligned as shown in FIG. 1 willonly weakly attenuate this reflection. In certain applications, this reflection will beacceptable. In general, however, the reflective polarizer will preferably haveminimal reflection in the second polarization over the range of optical angles used bythe device (nominally +/- 45 degrees for a TFT or STN liquid crystal display). Ingeneral it is preferred that the reflection of the reflective polarizer of linearlypolarized light of the second polarization be less than 20%, more preferrably less than10%, and most preferrably less than 5%. This reflectivity is the average for thewavelength range and use angle range of interest for specific or general applications.It is also preferred that the reflectivity of the reflective polarizer for linearly polarizedlight of the first polarization be preserved over angles orthogonal to the extinctionaxis relative to the normal direction. Preferred is that the reflectivity of the firstpolarization is reduced to no less than 30% at the maximum angle of interest, morepreferred is 60%, and most preferred is that the reflectivity be maintained or increaseat off-normal angles over the range of angles of interest.
• The reflective and dichroic polarizers may be various combinations of broadband and narrow band polarizers. For example, a narrow band reflective polarizermay be combined with a dichroic polarizer with extinction over the same band range.This combination can be used to produce polarizers in the red, green, blue, cyan,magenta, or yellow bands with higher extinction and superior color definitioncompared to a colored dichroic polarizer. Other combinations include the use of abroad band reflective polarizer with dichroic polarizers with nonuniform extinction inthe visible spectrum. For example, certain polyvinyl alcohol/iodine polarizers have excellent extinction in the green and red portion of the spectrum, and less in the blue.This polarizer can be combined with a broad band reflective polarizer in order toprovide good extinction at blue wavelengths. Nonuniform optical extinction may alsobe useful for increasing the optical performance of the combined polarizers. Forexample, the maximum radiometric transmission from the combination of reflectiveand dichroic polarizers may be obtained with minimum luminous reflectivity by usinga dichroic polarizer with relatively high absorption in the green and less absorption inthe blue and red. Insufficient extinction in the reflective polarizer at normal andoff-normal angles may also be compensated by increasing the extinction of thedichroic polarizer in the necessary spectral regions. For example, a reflectivepolarizer that has insufficient extinction for red light of the second polarization atoff-normal angles can be compensated by using a dichroic polarizer with relativelyred high extinction.
• Dichroic polarizer 11 can be incorporated intooptical polarizer 10 by placingthe reflective and dichroic polarizers in the same optical path or by laminating themtogether with an adhesive.Dichroic polarizer 11 can be incorporated withreflectivepolarizer 12 before orientation by extruding or laminating at least one layer of amixture of dichroic dyestuff in polymer onto the multilayer cast film, by a dichroicdyestuff added to the polymer resin of one or more of the skin layers of themultilayer reflective polarizer, or by adding resin to one or more layers in themultilayer stack. Multilayer extrusion techniques also allow the ability to tailor thedistribution of dichroic dye within the individual layers making up the optical stack.This may allow the dye to be located in regions of greatest utility. For example, adye may be preferably concentrated in regions of maximum or minimum "E" fieldintensity within the optical stack. By appropriate choice of the dichroic dyestuff andpolymer matrix, stretching the resulting film will simultaneously produce the dichroicand reflective polarizers in the proper orientation.
• Anthraquinone and azo dyes may be used as the dichroic dye, as well as otherdichroic dye materials. In some applications the dye does not have to be highlydichroic when oriented. Applications requiring relatively high absorption of both polarizations, for example, sunglasses or in displays requiring reduced glare, can usea less dichroic, or non-dichroic dye.
• Thedichroic polarizer 11 may be incorporated into one or both sides of areflective polarizer by coating a solution of polyvinyl alcohol onto the cast(unoriented) multilayer film and simultaneously forming the multilayer reflectivepolarizer and the dichroic polarizer. The cast film can be primed for adhesion beforecoating by solution coating on an inorganic or polymeric primer layer, coronatreatment, or by physical treatment. A suitable solution based primer for thisapplication are water soluble copolyesters commonly used for priming polyethyleneterephthlate films such as described in U.S. patent 4,659,523. The polyvinyl alcoholcoating solution should contain between 2 and 20% polymer in water based onweight, with the preferred concentration being between 5 and 15%. The polyvinylalcohol should have a degree of hydrolysis of between 95 and 100%, preferablybetween 97 and 99.5%. The coating weight should range from 2 to 80 grams persquare meter. The polyvinyl alcohol coated cast film is then stretched at elevatedtemperatures to develop oriented polyvinyl alcohol and the multilayer reflectivepolarizer. This temperature is preferably above the glass transition temperature ofleast one of the components of the multilayer reflective polarizer. In general, thetemperature should be between 80 and 160°C, preferably between 100 and 160°C.The film should be stretched from 2 to 10 times the original dimension. Preferably,the film will be stretched from 3 to 6 times the original dimension. The film may beallowed to dimensionally relax in the cross-stretch direction from the naturalreduction in cross-stretch (equal to the square root of the stretch ratio) to beingconstrained (i.e., no substantial change in cross-stretch dimensions). The film maybe stretched in the machine direction, as with a length orienter, in width using atenter, or at diagonal angles. The oriented polyvinyl alcohol coating is then stainedwith either iodine based staining solutions, dye based staining solutions, orcombinations of the two solutions and stabilized if necessary with suitable solutionssuch as boric acid and borax in water. Such staining and fixing techniques are knownin the art. After drying the film, the dichroic polarizer can be protected byadhesively laminating on a protective film such as cellulose based polymers, acrylate polymers, polycarbonate polymers, solution based or radiation cured acrylate basedadhesive or non-adhesive coatings, polyethylene terephthalate or other polyester basedfilms, or an additional sheet of reflective polarizer film. In cases where the state ofpolarized light rays entering or exiting thepolarizer 10 from the dichroic polarizerside is not critical, birefringent polymers such as biaxially oriented polyethyleneterephthalate may be used as the protective layer.
• A dichroic polarizer suitable for use in this invention is described in U.S.Patents 4,895,769 and 4,659,523. The polarizers described in these patents may becombined with the reflective polarizer preferably with the polyvinyl alcohol side ofthe polarizer adhesively bonded to the reflective polarizer. The dichroic polarizermay be made from relatively thin polyvinyl alcohol coatings (i.e., less than 4.5 gramsper square meter). Thin coatings will have less absorption of the polarizationperpendicular to the stretch direction, yet still have good extinction in firstpolarization when the high transmission axis is aligned with the high transmission axisof a reflective polarizer. Thin coatings are also faster to process.
• Other optical films may be attached to or used in the optical path of thepolarizer 10 for particular applications. Examples of these optical films includecircular or elliptical diffusers that either preserve or randomize polarization,hardcoated films, antireflective films, textured antiglare films, compensation films orstructures (used for example in liquid crystal displays), and optical retarderscommonly used to convert linear to elliptically or circularly polarized light.
• The preferred "A" layer of the multilayer reflective polarizer is a crystallinenaphthalene dicarboxilic acid polyester such as polyethylene naphthalate (PEN), andthe preferred "B" layer is a copolyester of naphthalene dicarboxilic acid andterephthalic acid (CoPEN). PEN and a 70 part naphthalate/30 part terephthalatecopolyester (CoPEN) can be synthesized in standard polyester resin kettles usingethylene glycol as the diol. A satisfactory 204-layered polarizer was made byextruding PEN and CoPEN in a 51-slot feed block and then employing two layerdoubling multipliers in series in the extrusion. The multipliers divide the extrudedmaterial exiting the feed block into two half-width flow streams, then stack thehalf-width flow streams on top of each other. Such multipliers are know in the art. The extrusion was performed at approximately 295°C. The PEN exhibited anintrinsic viscosity of 0.50 dl/g and the CoPEN exhibited an intrinsic viscosity of0.60 dl/g. The extrusion rate for the PEN material was 22.5 lb./hr and the extrusionrate for the CoPEN was 16.5 lb./hr. The cast web was approximately 0.0038 inchesin thickness and was uniaxially stretched at a 5:1 ratio in a longitudinal direction withthe sides restrained at an air temperature of 140°C during stretching. Except forexterior skin layers, all PEN/CoPEN layer pairs were designed to be 1/2 wavelengthoptical thickness for a design wavelength of 550 nm.
• Two 204-layer polarizers made as described above were then hand laminatedusing an adhesive. Preferably the refractive index of the adhesive should match theindex of the isotropic CoPEN layer.
• The polarizer of this invention has at least one dichroic polarizer and onereflective polarizer sections (as shown in FIG. 1). Other combinations are alsosuitable, including polarizers having either dichroic/reflective/dichroic sections orreflective/dichroic/reflective sections.
• FIG. 3 shows the combinedreflective polarizer 12 anddichroic polarizer 11 asused in a transmissive display.Liquid crystal module 52 switches the polarization oftransmitted light supplied bybacklight 54 through a conventionaldichroic polarizer53. In this mode, the reflective polarizer returns at least a portion of the light of thefirst polarization passed by theliquid crystal module 52 back into the backlight. Thislight may be recycled by the backlight and be used to increase the brightness of thedisplay.
• Fig. 4 shows the use of the combinedpolarizers 11 and 12 as the rearpolarizer in a transmissive display. In this mode, the reflective polarizer mayenhance the brightness of a display by returning the light of the first polarization thatwould ordinarily be absorbed by the rear dichroic polarizer in a conventional display.
• FIG. 5 shows combinedpolarizers 11 and 12 used as both the front and rearpolarizers in a display. The displays shown in FIG. 3, 4, and 5 can be used in atransflective mode by inserting a partial reflector between the backlight and the rearpolarizer, and can be used as a reflective display by replacing the backlight with areflective film.
• Mostliquid crystal module 52 such as those shown in Figs. 3, 4, and 5generally include a thin layer of liquid crystal material sandwiched between two glasslayers. To minimize parallax, the configuration shown in Fig. 6 can be used. Therethe combinedpolarizers 11 and 12 are located between theliquid crystal 56 and glasslayers 58 and 59 of theliquid crystal module 52. By locating the combined polarizersin this manner, parallax which may be otherwise introduced in varying degreesdepending upon the thickness of the glass layers is eliminated.
Example 1
• A Polaroid Corporation model number HN-38 dichroic polarizing film wasplaced against the multilayer reflective polarizer formed as discussed above. Thepolarizers were aligned for maximum transmission of one polarization. Thecombination of the dichroic and reflective polarizers eliminated visible iridescence ofthe reflective polarizing film when viewed in transmission in either direction. Thedichroic polarizer also eliminated reflected visible iridescence from the reflectivepolarizer when viewed through the dichroic polarizer. This example shows that thecombination of a dichroic polarizer with a reflective polarizer improves the cosmeticuniformity of the reflective polarizer.
Example 2
• The reflectivity and transmissivity of the optical polarizer of Example 1 wasmeasured with a Lambda 9 spectrophotometer at 550 nm using a sample beampolarized with a Melles-Griot dichroic polarizer model number 03-FPG-009.Reflectivity measurements were made using an integrating sphere. Separatereflectivity measurements were made with the samples backed first with a whitediffuse reflector and then with a black backing. The transmissivity of the combinedpolarizers was 65.64% when aligned in the spectrophotometer for maximumtransmission, and 0.05% when aligned for minimum transmission. When the dichroicpolarizer was facing the integrating sphere and an absorbing backing was used, thereflectivity of the combined polarizers was 13.26% when aligned for maximumreflectivity and 4.37% when aligned for minimum reflectivity. The maximum and minimum reflectivity of the combined polarizers when the reflective polarizer wasfacing the integrating sphere was 99.22% and 16.58%, respectively. The abovemeasurements were repeated with a white reflection standard behind the sample. Thereflectivity of the combined polarizers with the dichroic polarizer facing theintegrating sphere was 47.47% when aligned for maximum reflectivity, and 4.41%when aligned for minimum reflectivity. The maximum and minimum reflectivity ofthe combined polarizers when the reflective polarizer was facing the integratingsphere was 99.32% and 36.73%, respectively. This example shows that thecombination of the two polarizers effectively renders one side of the reflectivepolarizer antireflected without substantially affecting the reflectivity of the other sideof the reflective polarizer .
Example 3
• The transmission of Polaroid Corporation model HN-38 dichroic polarizingfilm and the reflective polarizer of example 1 were measured at 430 nm using theprocedure described in example 2. The transmission of the dichroic polarizer withthe sample cross polarized to the sample beam was 0.63%. The transmission of thereflective polarizer under the same conditions was 48%. The transmission of thecombination of the two polarizers aligned for minimum transmission was 0.31%.This example shows that the extinction of a dichroic polarizer can be increased byincluding a reflective polarizer in the optical path.
• Although the preferredreflective polarizer body 12 has been described as amultilayer stack of polymeric materials, it shall be understood that other reflectivepolarizers could be substituted therefore without departing from the scope of thepresent invention. Other reflective polarizers include cholesteric liquid crystalpolarizers using an optical retarder placed between the reflective polarizer anddichroic polarizer, tilted optic prismatic and non-prismatic multilayer polarizers, andfirst-order diffractive polarizers.
Optical Behavior of Multilayer Stacks
• The optical behavior of a multilayer stack such as that shown above willnow be described in more general terms. The multilayer stack can includehundreds or thousands of layers, and each layer can be made from any of anumber of different materials. The characteristics which determine the choice ofmaterials for a particular stack depend upon the desired optical performance ofthe stack.
• The stack can contain as many materials as there are layers in the stack.For ease of manufacture, preferred optical thin film stacks contain only a fewdifferent materials. For purposes of illustration, the present discussion willdescribe multilayer stacks including two materials.
• The boundaries between the materials, or chemically identical materialswith different physical properties, can be abrupt or gradual. Except for somesimple cases with analytical solutions, analysis of the latter type of stratifiedmedia with continuously varying index is usually treated as a much largernumber of thinner uniform layers having abrupt boundaries but with only a smallchange in properties between adjacent layers.
• The reflectance behavior at any angle of incidence, from any azimuthaldirection, is determined by the indices of refraction in each film layer of the filmstack. If we assume that all layers in the film stack receive the same processconditions, then we need only look at a single interface of a two component stackto understand the behavior of the entire stack as a function of angle.
• For simplicity of discussion, therefore, the optical behavior of a singleinterface will be described. It shall be understood, however, that an actualmultilayer stack according to the principles described herein could be made ofhundreds or thousands of layers. To describe the optical behavior of a singleinterface, such as the one shown in Fig.7, the reflectivity as a function of angleof incidence for s and p polarized light for a plane of incidence including thez-axis and one in-plane optic axis will be plotted.
• Fig. 7 shows two material film layers forming a single interface, withboth immersed in an isotropic medium of index no. For simplicity ofillustration, the present discussion will be directed toward an orthogonalmultilayer birefringent system with the optical axes of the two materials aligned,and with one optic axis (z) perpendicular to the film plane, and the other opticaxes along the x and y axis. It shall be understood, however, that the optic axesneed not be orthogonal, and that nonothorgonal systems are well within the spiritand scope of the present invention. It shall be further understood that the opticaxes also need not be aligned with the film axes to fall within the intended scopeof the present invention.
• The basic mathematical building blocks for calculating the optics of anystack of films of any thickness, are the well known Fresnel reflection andtransmission coefficients of the individual film interfaces. The Fresnelcoefficients predict the magnitude of the reflectivity of a given interface, at anyangle of incidence, with separate formulas for s and p-polarized light.
• The reflectivity of a dielectric interface varies as a function of angle ofincidence, and for isotropic materials, is vastly different for p and s polarizedlight. The reflectivity minimum for p polarized light is due to the so calledBrewster effect, and the angle at which the reflectance goes to zero is referred toas Brewster's angle.
• The reflectance behavior of any film stack, at any angle of incidence, isdetermined by the dielectric tensors of all films involved. A general theoreticaltreatment of this topic is given in the text by R.M.A. Azzam and N.M. Bashara,"Ellipsometry and Polarized Light", published by North-Holland, 1987. Theresults proceed directly from the universally well known Maxwell's equations.
• The reflectivity for a single interface of a system is calculated by squaringthe absolute value of the reflection coefficients for p and s polarized light, givenbyequations 1 and 2, respectively.Equations 1 and 2 are valid for uniaxialorthogonal systems, with the axes of the two components aligned.
• 1)rpp =n2z * n2o √(n1z2 - no2sin2) - n1z * n1o √(n2z2 - no2sin2)n2z * n2o √(n1z2 - no2sin2) + n1z * n1o √(n2z2 - no2sin2)
• 2)rss =√(n1o2 - no2sin2) - √(n2o2 - no2sin2)√(n1o2 - no2sin2) + √(n2o2 - no2sin2)
where  is measured in the isotropic medium.
• In a uniaxial birefringent system, n1x = n1y = n1o, and n2x = n2y =n2o.
• For a biaxial birefringent system,equations 1 and 2 are valid only forlight with its plane of polarization parallel to the x-z or y-z planes, as defined inFig. 7. So, for a biaxial system, for light incident in the x-z plane, n1o = n1xand n2o = n2x in equation 1 (for p-polarized light), and n1o = n1y and n2o =n2y in equation 2 (for s-polarized light). For light incident in the y-z plane, n1o= n1y and n2o = n2y in equation 1 (for p-polarized light), and n1o = n1x andn2o = n2x in equation 2 (for s-polarized light).
• Equations 1 and 2 show that reflectivity depends upon the indices ofrefraction in the x, y and z directions of each material in the stack. In anisotropic material, all three indices are equal, thus nx = ny = nz. Therelationship between nx, ny and nz determine the optical characteristics of thematerial. Different relationships between the three indices lead to three generalcategories of materials: isotropic, uniaxially birefringent, and biaxiallybirefringent.
• A uniaxially birefringent material is defined as one in which the index ofrefraction in one direction is different from the indices in the other twodirections. For purposes of the present discussion, the convention for describinguniaxial birefringent systems is for the condition nx = ny ≠ nz. The x and yaxes are defined as the in-plane axes and the respective indices, nx and ny, willbe referred to as the in-plane indices.
• One method of creating a uniaxial birefringent system is to biaxiallystretch a polymeric multilayer stack (e.g., stretched along two dimensions).Biaxial stretching of the multilayer stack results in differences between refractiveindices of adjoining layers for planes parallel to both axes thus resulting inreflection of light in both planes of polarization.
• A uniaxial birefringent material can have either positive or negativeuniaxial birefringence. Positive uniaxial birefringence occurs when the z-indexis greater than the in-plane indices (nz > nx and ny). Negative uniaxialbirefringence occurs when the z-index is less than the in-plane indices (nz < nxand ny).
• A biaxial birefringent material is defined as one in which the indices ofrefraction in all three axes are different, e.g., nx ≠ ny ≠nz. Again, the nx andny indices will be referred to as the in-plane indices. A biaxial birefringentsystem can be made by stretching the multilayer stack in one direction. In otherwords the stack is uniaxially stretched. For purposes of the present discussion,the x direction will be referred to as the stretch direction for biaxial birefringentstacks.
Uniaxial Birefringent Systems (Mirrors)
• The optical properties of uniaxial birefringent systems will now bediscussed. As discussed above, the general conditions for a uniaxial birefringentmaterial are nx = ny ≠ nz. Thus if eachlayer 102 and 104 in Fig. 7 isuniaxially birefringent, n1x = n1y and n2x = n2y. For purposes of the presentdiscussion, assume thatlayer 102 has larger in-plane indices thanlayer 104, andthat thus n1 > n2 in both the x and y directions. The optical behavior of auniaxial birefringent multilayer system can be adjusted by varying the values ofn1z and n2z to introduce different levels of positive or negative birefringence.
• Equation 1 described above can be used to determine the reflectivity of asingle interface in a uniaxial birefringent system composed of two layers such asthat shown in Fig.7.Equation 2, for s polarized light, is easily shown to beidentical to that of the simple case of isotropic films (nx = ny = nz), so we need only examineequation 1. For purposes of illustration, some specific, althoughgeneric, values for the film indices will be assigned. Let n1x = n1y = 1.75,n1z = variable, n2x = n2y = 1.50, and n2z = variable. In order to illustratevarious possible Brewster angles in this system, no = 1.60 for the surroundingisotropic media.
• Fig. 8 shows reflectivity versus angle curves for p-polarized light incidentfrom the isotropic medium to the birefringent layers, for cases where n1z isnumerically greater than or equal to n2z (n1z ≥ n2z). The curves shown inFig. 8 are for the following z-index values: a) n1z =1.75, n2z = 1.50; b) n1z= 1.75, n2z = 1.57; c) n1z = 1.70, n2z = 1.60; d) n1z = 1.65, n2z = 1.60;e) n1z = 1.61, n2z = 1.60; and f) n1z = 1.60 = n2z. As n1z approaches n2z,the Brewster angle, the angle at which reflectivity goes to zero, increases.
  Curves a-e are strongly angular dependent. However, when n1z = n2z (curvef), there is no angular dependence to reflectivity. In other words, the reflectivityfor curve f is constant for all angles of incidence. At that point,equation 1reduces to the angular independent form: (n2o - n1o)/(n2o + n1o). When n1z= n2z, there is no Brewster effect and there is constant reflectivity for all anglesof incidence.
• Fig. 9 shows reflectivity versus angle of incidence curves for cases wheren1z is numerically less than or equal to n2z. Light is incident from isotropicmedium to the birefringent layers. For these cases, the reflectivitymonotonically increases with angle of incidence. This is the behavior that wouldbe observed for s-polarized light. Curve a in Fig. 9 shows the single case for spolarized light. Curves b-e show cases for p polarized light for various values ofnz, in the following order: b) n1z =1.50, n2z = 1.60; c) n1z = 1.55, n2z =1.60; d) n1z =1.59, n2z = 1.60; and e) n1z = 1.60 = n2z. Again, when n1z= n2z (curve e), there is no Brewster effect, and there is constant reflectivity forall angles of incidence.
• Fig. 10 shows the same cases as Fig. 8 and 9 but for an incident mediumof index no = 1.0 (air). The curves in Fig. 10 are plotted for p polarized light ata single interface of a positive uniaxial material of indices n2x = n2y = 1.50, n2z = 1.60, and a negative uniaxially birefringent material with n1x = n1y =1.75, and values of n1z, in the following order, from top to bottom, of: a) 1.50;b) 1.55; c) 1.59; d) 1.60; f) 1.61; g) 1.65; h) 1.70; and i) 1.75. Again, aswas shown in Figs. 8 and 9, when the values of n1z and n2z match (curve d),there is no angular dependence to reflectivity.
• Figs. 8, 9 and 10 show that the cross-over from one type of behavior toanother occurs when the z-axis index of one film equals the z-axis index of theother film. This is true for several combinations of negative and positiveuniaxially birefringent, and isotropic materials. Other situations occur in whichthe Brewster angle is shifted to larger or smaller angles.
• Various possible relationships between in-plane indices and z-axis indicesare illustrated in Figs. 11, 12 and 13. The vertical axes indicate relative values ofindices and the horizontal axes are used to simply separate the variousconditions. Each Figure begins at the left with two isotropic films, where thez-index equals the in-plane indices. As one proceeds to the right, the in-planeindices are held constant and the various z-axis indices increase or decrease,indicating the relative amount of positive or negative birefringence.
• The case described above with respect to Figs. 8, 9,and 10 is illustratedin Fig. 11. The in-plane indices of material one are greater than the in-planeindices of material two,material 1 has negative birefringence (n1z less thanin-plane indices), and material two has positive birefringence (n2z greater thanin-plane indices). The point at which the Brewster angle disappears andreflectivity is constant for all angles of incidence is where the two z-axis indicesare equal. This point corresponds to curve f in Fig. 8, curve e in Fig. 9 or curved in Fig. 10.
• In Fig. 8, material one has higher in-plane indices than material two, butmaterial one has positive birefringence and material two has negativebirefringence. In this case, the Brewster minimum can only shift to lower valuesof angle.
• Both Figs. 11 and 12 are valid for the limiting cases where one of the twofilms is isotropic. The two cases are where material one is isotropic and materialtwo has positive birefringence, or material two is isotropic and material one hasnegative birefringence. The point at which there is no Brewster effect is wherethe z-axis index of the birefringent material equals the index of the isotropicfilm.
• Another case is where both films are of the same type, i.e., both negativeor both positive birefringent. Fig. 13 shows the case where both films havenegative birefringence. However, it shall be understood that the case of twopositive birefringent layers is analogous to the case of two negative birefringentlayers shown in Fig. 13. As before, the Brewster minimum is eliminated only ifone z-axis index equals or crosses that of the other film.
• Yet another case occurs where the in-plane indices of the two materialsare equal, but the z-axis indices differ. In this case, which is a subset of all threecases shown in Figs. 11-13, no reflection occurs for s polarized light at anyangle, and the reflectivity for p polarized light increases monotonically withincreasing angle of incidence. This type of article has increasing reflectivity forp-polarized light as angle of incidence increases, and is transparent to s-polarizedlight. This article can be referred to, then, as a "p-polarizer".
• Those of skill in the art will readily recognize that the above describedprinciples describing the behavior of uniaxially birefringent systems can beapplied to create the desired optical effects for a wide variety of circumstances.The indices of refraction of the layers in the multilayer stack can be manipulatedand tailored to produce devices having the desired optical properties. Manynegative and positive uniaxial birefringent systems can be created with a varietyof in-plane and z-axis indices, and many useful devices can be designed andfabricated using the principles described here.
Biaxial Birefringent Systems (Polarizers)
• Referring again to Fig. 7, two component orthogonal biaxial birefringentsystems will now be described. Again, the system can have many layers, but anunderstanding of the optical behavior of the stack is achieved by examining theoptical behavior at one interface.
• A biaxial birefringent system can be designed to give high reflectivity forlight with its plane of polarization parallel to one axis, for all angles ofincidence, and simultaneously have low reflectivity for light with its plane ofpolarization parallel to the other axis at all angles of incidence. As a result, thebiaxial birefringent system acts as a polarizer, transmitting light of onepolarization and reflecting light of the other polarization. By controlling thethree indices of refraction of each film, nx, ny and nz, the desired polarizerbehavior can be obtained.
• The multilayer reflecting polarizer of PEN/coPEN described above is anexample of a biaxial birefringent system. It shall be understood, however, thatin general the materials used to construct the multilayer stack need not bepolymeric. Any materials falling within the general principles described hereincould be used to construct the multilayer stack.
• Referring again to Fig. 7, we assign the following values to the filmindices for purposes of illustration: n1x = 1.88, n1y = 1.64, n1z = variable,n2x = 1.65, n2y = variable, and n2z = variable. The x direction is referred toas the extinction direction and the y direction as the transmission direction.
• Equation 1 can be used to predict the angular behavior of the biaxialbirefringent system for two important cases of light with a plane of incidence ineither the stretch or the non-stretch directions. The polarizer is a mirror in onepolarization direction and a window in the other direction. In the stretchdirection, the large index differential of 1.88 - 1.65 = 0.23 in a multilayer stackwith hundreds of layers will yield very high reflectivities for s-polarized light.For p-polarized light the reflectance at various angles depends on the n1z/n2zindex differential.
• In most applications, the ideal reflecting polarizer has high reflectancealong one axis and zero reflectance along the other, at all angles of incidence. Ifsome reflectivity occurs along the transmission axis, and if it is different forvarious wavelengths, the efficiency of the polarizer is reduced, and color isintroduced into the transmitted light. Both effects are undesirable. This is causedby a large z-index mismatch, even if the in-plane y indices are matched. Theresulting system thus has large reflectivity for p, and is highly transparent to spolarized light. This case was referred to above in the analysis of the mirrorcases as a "p polarizer".
• Fig. 14 shows the reflectivity (plotted as -Log[1-R]) at 75° for p polarizedlight with its plane of incidence in the non-stretch direction, for an 800 layer stackof PEN/coPEN. The reflectivity is plotted as function of wavelength across thevisible spectrum (400 - 700 nm). The relevant indices for curve a at 550 nm aren1y =1.64, n1z = 1.52, n2y = 1.64 and n2z = 1.63. The model stack design isa simple linear thickness grade for quarterwave pairs, where each pair is 0.3%thicker than the previous pair. All layers were assigned a random thickness errorwith a gaussian distribution and a 5% standard deviation.
• Curve a shows high off-axis reflectivity across the visible spectrum alongthe transmission axis (the y-axis) and that different wavelengths experiencedifferent levels of reflectivity. Since the spectrum is sensitive to layer thicknesserrors and spatial nonuniformities, such as film caliper, this gives a biaxialbirefringent system with a very nonuniform and "colorful" appearance. Althougha high degree of color may be desirable for certain applications, it is desirable tocontrol the degree of off-axis color, and minimize it for those applicationsrequiring a uniform, low color appearance, such as LCD displays or other typesof displays.
• If the film stack were designed to provide the same reflectivity for allvisible wavelengths, a uniform, neutral gray reflection would result. However,this would require almost perfect thickness contorl. Instead, off-axis reflectivity, and off-axis color can be minimized by introducing an index mismatch to thenon-stretch in-plane indices (n1y and n2y) that create a Brewster condition offaxis, while keeping the s-polarization reflectivity to a minimum.
• Fig. 15 explores the effect of introducing a y-index mismatch in reducingoff-axis reflectivity along the transmission axis of a biaxial birefringent system.With n1z = 1.52 and n2z = 1.63 (Δnz = 0.11), the following conditions areplotted for p polarized light: a) n1y = n2y = 1.64; b) n1y = 1.64, n2y =1.62; c) n1y = 1.64, n2y = 1.66. Curve a shows the reflectivity where thein-plane indices n1y and n2y are equal. Curve a has a reflectance minimum at0°, but rises steeply after 20°. For curve b, n1y > n2y, and reflectivityincreases rapidly. Curve c, where n1y < n2y, has a reflectance minimum at38°, but rises steeply thereafter. Considerable reflection occurs as well for spolarized light for n1y ≠ n2y, as shown by curve d. Curves a -d of Fig. 15indicate that the sign of the y-index mismatch (n1y - n2y) should be the same asthe z-index mismatch (n1z- n2z) for a Brewster minimum to exist. For the caseof n1y = n2y, reflectivity for s polarized light is zero at all angles.
• By reducing the z-axis index difference between layers, the off axisreflectivity can be further reduced. If n1z is equal to n2z, Fig. 10 indicates thatthe extinction axis will still have a high reflectivity off-angle as it does at normalincidence, and no reflection would occur along the nonstretch axis at any anglebecause both indices are matched (e.g., n1y = n2y and n1z = n2z).
• Exact matching of the two y indices and the two z indices may not bepossible in some polymer systems. If the z-axis indices are not matched in apolarizer construction, a slight mismatch may be required for in-plane indices n1yand n2y. Another example is plotted in FIG. 16 assuming n1z = 1.56 and n2z =1.60 (Δnz = 0.04), with the following y indices a) n1y = 1.64, n2y = 1.65; b)n1y = 1.64, n2y = 1.63. Curve c is for s-polarized light for either case. Curvea, where the sign of the y-index mismatch is the same as the z-index mismatch,results in the lowest off-angle reflectivity.
• The computed off-axis reflectance of an 800 layer stack of films at 75°angle of incidence with the conditions of curve a in Fig. 16 is plotted as curve bin Fig. 14. Comparison of curve b with curve a in Fig. 14 shows that there is farless off-axis reflectivity, and therefore lower perceived color, for the conditionsplotted in curve b. The relevant indices for curve b at 550 nm are n1y = 1.64,n1z = 1.56, n2y = 1.65 and n2z = 1.60.
• Fig. 17 shows a contour plot ofequation 1 which summarizes the off axisreflectivity discussed in relation to Fig. 7 for p-polarized light. The fourindependent indices involved in the non-stretch direction have been reduced totwo index mismatches, Δnz and Any. The plot is an average of 6 plots at variousangles of incidence from 0° to 75° in 15 degree increments. The reflectivityranges from 0.4 x 10-4 for contour a, to 4.0 x 10-4 for contour j, in constantincrements of 0.4 x 10 -4. The plots indicate how high reflectivity caused by anindex mismatch along one optic axis can be offset by a mismatch along the otheraxis.
• Thus, by reducing the z-index mismatch between layers of a biaxialbirefringent systems, and/or by introducing a y-index mismatch to produce aBrewster effect, off-axis reflectivity, and therefore off-axis color, are minimizedalong the transmission axis of a multilayer reflecting polarizer.
• It should also be noted that narrow band polarizers operating over anarrow wavelength range can also be designed using the principles describedherein. These can be made to produce polarizers in the red, green, blue, cyan,magenta, or yellow bands, for example.
Materials Selection and Processing
• With the above-described design considerations established, one ofordinary skill will readily appreciate that a wide variety of materials can be usedto form multilayer mirrors or polarizers according to the invention whenprocessed under conditions selected to yield the desired refractive indexrelationships. In general, all that is required is that one of the materials have adifferent index of refraction in a selected direction compared to the second material. This differential can be achieved in a variety of ways, includingstretching during or after film formation (e.g., in the case of organic polymers),extruding (e.g., in the case of liquid crystalline materials), or coating. Inaddition, it is preferred that the two materials have similar rheological properties(e.g., melt viscosities) such that they can be co-extruded.
• In general, appropriate combinations may be achieved by selecting, as thefirst material, a crystalline or semi-crystalline organic polymer and an organicpolymer for the second material as well. The second material, in turn, may becrystalline, semi-crystalline, or amorphous, or may have a birefringence oppositethat of the first material.
• Specific examples of suitable materials include polyethylene naphthalate(PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN),polyalkylene terephthalates (e.g., polyethylene terephthalate, polybutyleneterephthalate, and poly-1,4-cyclohexanedimethylene terephthalate), polyimides(e.g., polyacrylic imides), polyetherimides, atactic polystyrene, polycarbonates,polymethacrylates (e.g., polyisobutyl methacrylate, polypropylmethacrylate,polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,polybutylacrylate and polymethylacrylate), cellulose derivatives (e.g., ethylcellulose, cellulose acetate, cellulose propionate, cellulose acetate butyrate, andcellulose nitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinated polymers(e.g., perfluoroalkoxy resins, polytetrafluoroethylene, fluorinated ethylenepropylenecopolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene),chlorinated polymers (e.g., polyvinylidene chloride and polyvinylchloride),polysulfones, polyethersulfones, polyacrylonitrile, polyamides, silicone resins,epoxy resins, polyvinylacetate, polyether-amides, ionomeric resins, elastomers(e.g., polybutadiene, polyisoprene, and neoprene), and polyurethanes. Alsosuitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of 2,6-,1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, or esters thereof, with
• (a) terephthalic acid, or esters thereof; (b) isophthalic acid, or esters thereof; (c)phthalic acid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol diol); (f) alkane dicarboxylic acids; and/or (g)cycloalkane dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),copolymers of polyalkylene terephthalates (e.g., copolymers of terephthalic acid,or esters thereof, with (a) naphthalene dicarboxylic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or esters thereof; (d) alkaneglycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol diol); (f) alkanedicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids (e.g., cyclohexanedicarboxylic acid)), and styrene copolymers (e.g., styrene-butadiene copolymersand styrene-acrylonitrile copolymers), 4, 4' bibenzoic acid and ethylene glycol.In addition, each individual layer may include blends of two or more of theabove-described polymers or copolymers (e.g., blends of SPS and atacticpolystyrene).
• Particularly preferred combinations of layers in the case of polarizersinclude PEN/co-PEN, polyethylene terephthalate (PEI)/co-PEN, PEN/SPS,PET/SPS, PEN/Eastair, and PET/Eastair, where "co-PEN" refers to acopolymer or blend based upon naphthalene dicarboxylic acid (as describedabove) and Eastair is polycyclohexanedimethylene terephthalate commerciallyavailable from Eastman Chemical Co.
• Particularly preferred combinations of layers in the case of mirrorsinclude PET/Ecdel, PEN/Ecdel, PEN/SPS, PEN/THV, PEN/co-PET, andPET/SPS, where "co-PET" refers to a copolymer or blend based uponterephthalic acid (as described above), Ecdel is a thermoplastic polyestercommercially available from Eastman Chemical Co., and THV is afluoropolymer commercially available from 3M Co.
• The number of layers in the device is selected to achieve the desiredoptical properties using the minimum number of layers for reasons of economy.In the case of both polarizers and mirrors, the number of layers is preferably lessthan 10,000, more preferably less than 5,000, and (even more preferably) lessthan 2,000.
• As discussed above, the ability to achieve the desired relationships amongthe various indices of refraction (and thus the optical properties of the multilayerdevice) is influenced by the processing conditions used to prepare the multilayerdevice. In the case of organic polymers which can be oriented by stretching, thedevices are generally prepared by co-extruding the individual polymers to form amultilayer film and then orienting the film by stretching at a selectedtemperature, optionally followed by heat-setting at a selected temperature.Alternatively, the extrusion and orientation steps may be performedsimultaneously. In the case of polarizers, the film is stretched substantially inone direction (uniaxial orientation), while in the case of mirrors the film isstretched substantially in two directions (biaxial orientation).
• The film may be allowed to dimensionally relax in the cross-stretchdirection from the natural reduction in cross-stretch (equal to the square root ofthe stretch ratio) to being constrained (i.e., no substantial change in cross-stretchdimensions). The film may be stretched in the machine direction, as with alength orienter, in width using a tenter, or at diagonal angles.
• The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, and cross-stretchrelaxation are selected to yield a multilayer device having the desired refractiveindex relationship. These variables are inter-dependent; thus, for example, arelatively low stretch rate could be used if coupled with, e.g., a relatively lowstretch temperature. It will be apparent to one of ordinary skill how to select theappropriate combination of these variables to achieve the desired multilayerdevice. In general, however, a stretch ratio of 1:2-10 (more preferably 1:3-7) ispreferred in the case of polarizers. In the case of mirrors, it is preferred that thestretch ratio along one axis be in the range of 1:2-10 (more preferably 1:2-8, andmost preferably 1:3-7) and the stretch ratio along the second axis be in the rangeof 1:-0.5-10 (more preferably 1:1-7, and most preferably 1:3-6).
• Suitable multilayer devices may also be prepared using techniques such asspin coating (e.g., as described in Boese et al., J. Polym. Sci.: Part B, 30:1321(1992)) and vacuum deposition; the latter technique is particularly useful in thecase of crystalline polymeric organic and inorganic materials.
• The invention will now be described by way of the following examples.In the examples, because optical absorption is negligible, reflection equals 1minus tranmission (R = 1 - T).
Mirror Examples:
• PET:Ecdel, 601 A coextruded film containing 601 layers was made on asequential flat-film-making line via a coextrusion process. Polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt. % phenol/40wt. % dichlorobenzene) was delivered by one extruder at a rate of 75 pounds perhour and Ecdel 9966 (a thermoplastic elastomer available from EastmanChemical) was delivered by another extruder at a rate of 65 pounds per hour.PET was on the skin layers. The feedblock method (such as that described inU.S. Patent 3,801,429) was used to generate 151 layers which was passedthrough two multipliers producing an extrudate of 601 layers. U.S. Patent3,565,985 describes examplary coextrusion multipliers. The web was lengthoriented to a draw ratio of about 3.6 with the web temperature at about 210°F.The film was subsequently preheated to about 235°F in about 50 seconds anddrawn in the transverse direction to a draw ratio of about 4.0 at a rate of about6% per second. The film was then relaxed about 5% of its maximum width in aheat-set oven set at 400°F. The finished film thickness was 2.5 mil.
• The cast web produced was rough in texture on the air side, and providedthe transmission as shown in Figure 18. The % transmission for p-polarizedlight at a 60° angle (curve b) is similar the value at normal incidence (curve a)(with a wavelength shift).
• For comparison, film made by Mearl Corporation, presumably ofisotropic materials (see Fig. 19 )shows a noticeable loss in reflectivity forp-polarized light at a 60° angle (curve b, compared to curve a for normalincidence).
• PET:Ecdel, 151 A coextruded film containing 151 layers was made on asequential flat-film-making line via a coextrusion process. Polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt phenol/40wt. % dichlorobenzene) was delivered by one extruder at a rate of 75 pounds perhour and Ecdel 9966 (a thermoplastic elastomer available from EastmanChemical) was delivered by another extruder at a rate of 65 pounds per hour.PET was on the skin layers. The feedblock method was used to generate 151layers. The web was length oriented to a draw ratio of about 3.5 with the webtemperature at about 210°F. The film was subsequently preheated to about215°F in about 12 seconds and drawn in the transverse direction to a draw ratioof about 4.0 at a rate of about 25% per second. The film was then relaxed about5% of its maximum width in a heat-set oven set at 400°F in about 6 seconds.The finished film thickness was about 0.6 mil.
• The transmission of this film is shown in Figure 20. The % transmissionfor p-polarized light at a 60° angle (curve b) is similar the value at normalincidence (curve a) with a wavelength shift. At the same extrusion conditions theweb speed was slowed down to make an infrared reflecting film with a thicknessof about 0.8 mils. The transmission is shown in Fig. 21 (curve a at normalincidence, curve b at 60 degrees).
• PEN:Ecdel, 225 A coextruded film containing 225 layers was made byextruding the cast web in one operation and later orienting the film in alaboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) wasdelivered by one extruder at a rate of 18 pounds per hour and Ecdel 9966 (athermoplastic elastomer available from Eastman Chemical) was delivered by another extruder at a rate of 17 pounds per hour. PEN was on the skin layers.The feedblock method was used to generate 57 layers which was passed throughtwo multipliers producing an extrudate of 225 layers. The cast web was 12 milsthick and 12 inches wide. The web was later biaxially oriented using alaboratory stretching device that uses a pantograph to grip a square section offilm and simultaneously stretch it in both directions at a uniform rate. A 7.46 cmsquare of web was loaded into the stretcher at about 100°C and heated to 130°Cin 60 seconds. Stretching then commenced at 100%/sec (based on originaldimensions) until the sample was stretched to about 3.5 x 3.5. Immediately afterthe stretching the sample was cooled by blowing room temperature air on it.
• Figure 22 shows the optical response of this multilayer film (curve a atnormal incidence, curve b at 60 degrees). Note that the % transmission forp-polarized light at a 60° angle is similar to what it is at normal incidence (withsome wavelength shift).
• PEN:THV 500, 449 A coextruded film containing 449 layers was made byextruding the cast web in one operation and later orienting the film in alaboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) wasdelivered by one extruder at a rate of 56 pounds per hour and THV 500 (afluoropolymer available from Minnesota Mining and Manufacturing Company)was delivered by another extruder at a rate of 11 pounds per hour. PEN was onthe skin layers and 50% of the PEN was present in the two skin layers. Thefeedblock method was used to generate 57 layers which was passed through threemultipliers producing an extrudate of 449 layers. The cast web was 20 mils thickand 12 inches wide. The web was later biaxially oriented using a laboratorystretching device that uses a pantograph to grip a square section of film andsimultaneously stretch it in both directions at a uniform rate. A 7.46 cm squareof web was loaded into the stretcher at about 100°C and heated to 140°C in 60 seconds. Stretching then commenced at 10%/sec (based on original dimensions)until the sample was stretched to about 3.5x3.5. Immediately after the stretchingthe sample was cooled by blowing room temperature air at it.
• Figure 23 shows the transmission of this multilayer film. Again, curve ashows the response at normal incidence, while curve b shows the response at 60degrees.
Polarizer Examples:
• PEN:CoPEN, 449-Low Color A coextruded film containing 449 layers wasmade by extruding the cast web in one operation and later orienting the film in alaboratory film-stretching apparatus. Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.56 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) wasdelivered by one extruder at a rate of 43 pounds per hour and a CoPEN(70mol% 2,6 NDC and 30 mol% DMT) with an intrinsic viscosity of 0.52(60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by another extruderat a rate of 25 pounds per hour. PEN was on the skin layers and 40% of thePEN was present in the two skin layers. The feedblock method was used togenerate 57 layers which was passed through three multipliers producing anextrudate of 449 layers. The cast web was 10 mils thick and 12 inches wide.The web was later uniaxially oriented using a laboratory stretching device thatuses a pantograph to grip a square section of film and stretch it in one directionwhile it is constrained in the other at a uniform rate. A 7.46 cm square of webwas loaded into the stretcher at about 100°C and heated to 140°C in 60 seconds.Stretching then commenced at 10%/sec (based on original dimensions) until thesample was stretched to about 5.5 x 1. Immediately after the stretching thesample was cooled by blowing room temperature air at it.
• Figure 24 shows the transmission of this multilayer film. Curve a showstransmission of p-polarized light at normal incidence, curve b shows transmissionof p-polarized light at 60° incidence, and curve c shows transmission ofs-polarized light at normal incidence. Note the very high transmission ofp-polarized light at both normal and 60° incidence (85-100%). Transmission is higher for p-polarized light at 60° incidence because the air/PEN interface has aBrewster angle near 60°, so the tranmission at 60° incidence is nearly 100%.Also note the high extinction of s-polarized light in the visible range(400-700nm) shown by curve c.
• PEN:CoPEN, 601-High Color A coextruded film containing 601 layers wasproduced by extruding the web and two days later orienting the film on adifferent tenter than described in all the other examples. PolyethyleneNaphthalate (PEN) with an Intrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40wt. % dichlorobenzene) was delivered by one extruder at a rate of 75 pounds perhour and CoPEN (70mol% 2,6 NDC and 30 mol% DMT) with an IV of0.55 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by anotherextruder at a rate of 65 pounds per hour. PEN was on the skin layers. Thefeedblock method was used to generate 151 layers which was passed through twomultipliers producing an extrudate of 601 layers. U.S. Patent 3,565,985describes similar coextrusion multipliers. All stretching was done in the tenter.The film was preheated to about 280°F in about 20 seconds and drawn in thetransverse direction to a draw ratio of about 4.4 at a rate of about 6% persecond. The film was then relaxed about 2% of its maximum width in a heat-setoven set at 460°F. The finished film thickness was 1.8 mil.
• The transmission of the film is shown in Figure 25. Curve a showstransmission of p-polarized light at normal incidence, curve b shows transmissionof p-polarized light at 60° incidence, and curve c shows transmission ofs-polarized light at normal incidence. Note the nonuniform transmission ofp-polarized light at both normal and 60° incidence. Also note the non-uniformextinction of s-polarized light in the visible range (400-700nm) shown bycurve c.
• PET:CoPEN, 449 A coextruded film containing 449 layers was made byextruding the cast web in one operation and later orienting the film in alaboratory film-stretching apparatus. Polyethylene Terephthalate (PET) with anIntrinsic Viscosity of 0.60 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) wasdelivered by one extruder at a rate of 26 pounds per hour and CoPEN (70mol%2,6 NDC and 30 mol% DMT) with an intrinsic viscosity of 0.53 (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered by another extruder at a rate of24 pounds per hour. PET was on the skin layers. The feedblock method wasused to generate 57 layers which was passed through three multipliers producingan extrudate of 449 layers. U.S. Patent 3,565,985 describes similar coextrusionmultipliers. The cast web was 7.5 mils thick and 12 inches wide. The web waslater uniaxially oriented using a laboratory stretching device that uses apantograph to grip a square section of film and stretch it in one direction while itis constrained in the other at a uniform rate. A 7.46 cm square of web wasloaded into the stretcher at about 100°C and heated to 120°C in 60 seconds.Stretching then commenced at 10%/sec (based on original dimensions) until thesample was stretched to about 5.0x1. Immediately after the stretching the samplewas cooled by blowing room temperature air at it. The finished film thicknesswas about 1.4 mil. This film had sufficient adhesion to survive the orientationprocess with no delamination.
• Figure 26 shows the transmission of this multilayer film. Curve a showstransmission of p-polarized light at normal incidence, curve b shows transmissionof p-polarized light at 60° incidence, and curve c shows transmission ofs-polarized light at normal incidence. Note the very high transmission ofp-polarized light at both normal and 60° incidence (80-100%).
• PEN:coPEN, 601 A coextruded film containing 601 layers was made on a.sequential flat-film-making line via a coextrusion process. Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt % Phenol plus40 wt % dichlorobenzene) was delivered by on extruder at a rate of 75 poundsper hour and the coPEN was delivered by another extruder at 65 pounds per hour. The coPEN was a copolymer of 70mole % 2,6 naphthalene dicarboxylatemethyl ester, 15 % dimethyl isophthalate and 15% dimethyl terephthalate withethylene glycol. The feedblock method was used to generate 151 layers. Thefeedblock was designed to produce a gradient distribution of layers with a rationof thickness of the optical layers of 1.22 for the PEN and 1.22 for the coPEN.PEN skin layers were coextruded on the outside of the optical stack with a totalthickness of 8% of the coextruded layers. The optical stack was multiplied bytwo sequential multipliers. The nominal multiplication ratio of the multiplierswere 1.2 and 1.22, respectively. The film was subsequently preheated to 310°Fin about 40 seconds and drawn in the transverse direction to a draw ratio of about5.0 at a rate of 6% per second. The finished film thickness was about 2 mils.
• Figure 27 shows the transmission for this multilayer film. Curve a showstransmission of p-polarized light at normal incidence, curve b shows transmissionof p-polarized light at 60° incidence, and curve c shows transmission ofs-polarized light at normal incidence. Note the very high transmission ofp-polarized light at both normal and 60° incidence (80-100%). Also note the veryhigh extinction of s-polarized light in the visible range (400-700nm) shown bycurve c. Extinction is nearly 100% between 500 and 650nm.
• For those examples using the 57 layer feedblock, all layers were designedfor only one optical thickness (1/4 of 550nm), but the extrusion equipmentintroduces deviations in the layer thicknesses throughout the stack resulting in afairly broadband optical response. For examples made with the 151 layerfeedblock, the feedblock is designed to create a distribution of layer thicknessesto cover a portion of the visible spectrum. Asymmetric multipliers were thenused to broaden the distribution of layer thicknesses to cover most of the visiblespectrum as described in U.S. Patents 5,094,788 and 5,094,793.
• Although the present optical polarizer has been described with reference to thepreferred embodiment, those skilled in the art will readily appreciate that otherembodiments may be utilized and changes made in form and detail without departingfrom the scope of the present invention.

Claims (12)

1. An optical polarizer comprising:

   a reflective polarizer (12), including at least one birefringent material,which transmits light having a first polarization and reflectslight which does not have the first polarization; and

   a dichroic absorbing polarizer (11) in close proximity to one side ofthe reflective polarizer (12) to produce antireflection on the oneside of the reflective polarizer (12).
2. The optical polarizer of claim 1 wherein the reflective polarizer (12)has a first transmission axis and wherein the dichroic polarizer (11)has a second transmission axis, and further wherein the first transmissionaxis is aligned with the second transmission axis.
3. The optical polarizer of claim 1 wherein the reflective polarizer (12)comprises a multilayer stack of at least two materials, at least one ofwhich is birefringent.
4. The optical polarizer of claim 3 wherein the dichroic polarizer (11) isincorporated into at least one of the layers in the multilayer stack.
5. The optical polarizer of claim 3 wherein at least one of the materialsis polymeric.
6. The optical polarizer of claim 5 wherein the birefringent polymericmaterial is a poly(ethylene naphthalate).
7. The optical polarizer of claim 6 wherein the multilayer stack comprisesalternating layers of two polymeric materials.
8. The optical polarizer of claim 7 wherein the two polymeric materialsexhibit no refractive index difference for light (16, 19) of the firstpolarization, and further which exhibit a refractive index differencefor light (13, 17) having a polarization perpendicular to the first polarization.
9. The optical polarizer of claim 8 wherein the multilayer stack comprisesalternating layers of poly(ethylene naphthalate) and a copolyesterof a naphthalene dicarboxylic acid polyester.
10. The optical polarizer of claim 1 wherein the dichroic polarizer (11) isbonded to the reflective polarizer (12).
11. A display device, comprising an optical polarizer (10) as defined Inany of claims 1 to 10 and a light source arranged to illuminate atleast one outside facing surface of the optical polarizer (10).
12. A display comprising a liquid crystal module (52), a backlight (54)and the optical polarizer (11, 12) of claim 1 between the liquidcrystal module (52) and the backlight (54) with the dichroic absorbingpolarizer (11) toward the liquid crystal module (52),wherein the dichroic absorbing polarizer (11) provides antireflectionfrom a viewing side of the display.

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